Vinyl acetate latexes have been used widely as binders in the manufacture of paints, textiles, papers, pressure sensitive adhesives and the like. Such applications require latexes which possess certain properties, such as film clarity, film integrity, high gloss, good adhesion, etc.
Some monomers or combinations of monomers are known to frequently produce latexes in a certain particle size range, e.g., acrylic latexes are usually small to medium size (&lt;4000 .ANG.) and vinyl acetate containing latexes are usually large (&gt;4000 .ANG.).
The free radical catalyzed polymerization of vinyl acetate copolymers in an emulsion, usually requires a so-called protective colloid (polyvinyl alcohol, hydroxyethyl cellulose (HEC), polyacrylic acids, etc.) to stabilize the latexes. There are several disadvantages to such systems:
The presence of HEC in a vinyl acetate latex during polymerization, increases the viscosity both during polymerization and of the finished latex. High viscosity during polymerization can cause poor agitation which produces poor mixing and heat transfer. High viscosity of the finished latex can make filtration and transfer from reactor to storage, etc., slow.
The high viscosity can be an even bigger problem when preparing and handling very small particle latexes of the type prepared in this patent. At a constant non-volatile content, the latex viscosity increases as the particle size decreases. With HEC present, the viscosity can become unacceptably high at very small particle size.
Another disadvantage experienced when HEC is present, is batch-to-batch reproducibility. The HEC polymer is susceptible to oxidative cleavage by the persulfate used to initiate polymerization. Unfortunately the extent of degradation may vary considerably from batch to batch so the particle size can vary accordingly. Without the HEC present, as in our invention, the HEC degradation and related particle size effect are not of concern.
Due to the hydrophilic nature of these protective colloids, the particle size of vinyl acetate latexes stabilized by these colloids are very large (2000 to 10,000 .ANG.) and also the latex films are very water-sensitive. Large particle size latexes suffer the properties of low gloss, poor film integrity, poor penetration power, and poor pigment binding capacity, etc. Therefore, the development of small particle size vinyl acetate latexes with particle size less than 1000 .ANG. is highly desirable.
The preparation of ultrafine styrene-acrylic and all-acrylic latexes with particle size less than 1000 .ANG. is not unusual, but for vinyl acetate latexes, it is very rare. In this invention, a novel process has been developed to prepare very stable, and clean vinyl acetate latexes having an average particle size below 1000 .ANG. and preferably from 400 to 700 .ANG. without protective colloids. Ionic comonomers, e.g., sodium vinyl sulfonate. have been used widely in preparing water-soluble polymers, dispersants and wetting agents, and ion-exchange resins. The dyeability of fibers and the stability of polymer latexes can be improved by incorporating ionic monomers. U.S. Pat. Nos. 3,318,830 and 3,320,199 (1967) disclosed the preparation of vinyl acetate latexes containing sodium vinyl sulfonate, with alkyl sulfate and polyalkylene glycol ether as surfactant. The mean particle size was between 1800 .ANG. to 2200 .ANG.. BP No. 1,307,890 (1970) disclosed a process for the manufacture of vinyl ester polymer dispersions with particle size of 300 to 2000 .ANG.. The monomer also contained a small amount of acrylic acid, acrylamide, and sodium vinyl sulfonate (&lt;1%). The surfactant systems contained both anionic surfactants, e.g., sodium lauryl sulfate and nonionic surfactant, e.g., nonylphenol polyglycol ether (23 EO units). Sodium vinyl sulfonate was added all in the beginning, while acrylic acid and acrylamide were metered in with the monomer mixture.
In JK No. 79,103,497 and No. 79,103,498 (1979), superfine acrylic and vinyl acetate latexes were prepared by emulsion polymerization in the presence of anionic surfactant (sodium lauryl sulfate), redox catalyst (persulfate-thiosulfate), Cu (II) as accelerator, and thickening preventive (ammonium hydroxide). In U.S. Pat. No. 4,193,902 (1980), a process for making fine particle size styrene-acrylic latexes having particle size of 100 to 600 .ANG. was disclosed. Amides of unsaturated carboxylic acids, especially acrylamide or methacrylamide, promote the formation of finely distributed dispersions, when they are added to the batch during the polymerization simultaneously with the other monomers. Other related patents include U.S. Pat. No. 4,089,999 (1978); GP No. 2,556,327 (1977); BP No. 1,114,133 (1968); U.S. Pat. No. 3,740,367 (1968); Ger. Offen. DE No. 3,402,447 (1985); U.S. Pat. No. 3,965,032; Ger. Offen. No. 2,540,468 (1977); Ger. Offen. No. 2,309,368 (1974); U.S. Pat. No. 3,306,871 (1967).
It is known to use 2-acrylamido-2-methylpropane sulfonic acid, sodium salt (AMPS) as a major monomer component in preparing water-soluble polymers. U.S. Pat. No. 4,544,719 teaches such polymers formed from acrylamide and N,N-diallylacetamide. U.S. Pat. No. 4,544,722 teaches terpolymers formed from N-vinylpyrrolidone and acrylonitrile. (See also U.S. Pat. No. 4,502,965; 4,502,966; 4,451,631; 3,336,269; 4,451,631.)